Patent GB-220027-A (GB)
Title: Improved gear and method and machine for generating same
IPC: B23F21/18
Filed: 1923-04-30
Abstract: 220,027. Trbojevich, N. April 30, 1923. Gear-cutting; milling-cutters. - A system of bevel gears is based upon a crown wheel form which has non-parallel teeth which are longitudinally of modified involute curvature, a modified involute being defined mathematically as a tangential curve of the common involute of a circle. Thus in Fig. 3 at a point P on the involute described from the base circle of radius a a, tangent is drawn and a distance p measured along this tangent locates a point p<1> on a modified involute. A hob and a machine for cutting such a system of gears are also described. A positive value of the modification p gives a curve defined as an extended involute (Fig. 3) while a negative value gives an abridged involute. It is shown that the Archimedean spiral is a special case of the abridged involute when p=-a. It is then deduced that two series of modified involutes may be tangentially superposed upon a common rack generator if their respective modifications are of the same absolute value and are of the same pitch. Fig. 4 shows a series o: extended involutes generated from a line g offset from the generator g of the common in volutes by a distance po, the cutting hob being placed with its apex at a distance (a + po) from the apex J of the gear. Similar principles may be applied to the cutting of teeth of abridged involute form. A further advantage of the new system over teeth which are longitudinally o common involute form is that the radius o: curvature is greater at all points which facili. tates machining or grinding with rotary cutters The conical hob used must have a modification equal to that of the gears to be cut and have a cone angle sufficient to avoid interference. Further, the initial helix angle of the gears must not be greater than about 45‹. The length of the teeth should preferably be from 20 to 25 per cent of the pitch cone radius. The smallest practicable cone angle for the hob is shown to be about 25‹ but preferably the angle is 30‹ and such a hob will generate a width of face of 23 to 26 per cent of the pitch cone radius and a range of helical angles from 27‹ at the small end to 45‹ at the large end. To make the hob, a taper worm of constant pitch is first milled and then gashed lengthwise, to form cutting edges which are subsequently relieved and the cutter hardened and ground all over. The hob is shown in Fig. 15. A threaded ring W engages a threaded boss on the hob by which the latter is tightened on its arbor. The cutting teeth S are radial and are relieved perpendicularly to the side of the cone. The preferred pressure angle is 20‹. The relative positions of the hob and blank during cutting are shown in Figs. 13 and 14 and the arrangement of gearing to provide the necessary cutting movements is shown in Fig. 16. The hob is adjusted so that its apex is at a distance (a + p) from the centre line of the machine and on the right or left according as right or left handed gears are to be cut. The hob U is rotated from the main driving pulley 20 through bevel gears 21, spur gears 24 and bevel gears 27, the hob being mounted in a head 28 and adjustable in any desired angular position about the axis of gear 26 by bolts 30. The hob spindle is also adjustable lengthwise. The blank Z is driven at the required speed ratio through bevel gears 32, 34, differential gear 39, mitres 40, and change gears 44, the work head 49 being angularly adjusted in a horizontal plane about the apex 31 and longitudinally along its own axis. The feed of the hob is produced by the rotation of the cylinder 29 carrying the cutter head by means of worm gearing 51, feed change-gears 53, and worm-gear 54. compensating movement being imparted to the blank from the feed gears through change-gears 56 driving the differential sun gear 59. Specification 208.177 is referred to.